1. Field of the Invention
The present invention relates to a method of dicing a semiconductor substrate into, for example, microelectromechanical system (MEMS) chips.
2. Description of the Related Art
MEMS chips are generally diced from a silicon wafer by a dicing apparatus with a saw that cuts along a predetermined grid of scribe lines. The silicon wafer, which may be only a few hundred micrometers (μm) thick, is first taped with UV tape (a type of adhesive tape that loses adhesion on exposure to ultraviolet light). The dicing saw has a thin blade coated with diamond particles, which are harder than silicon. The diamond particles are held in a metal bonding material electro-deposited on the surface of the blade.
In an alternate method for use in dicing MEMS chips formed in a silicon-on-insulator (SOI) wafer, the supporting substrate layer of the wafer is etched away below the scribe lines; then the overlying oxide insulator film and silicon active layer are cut along the scribe lines with a laser beam, as disclosed in Japanese Patent Application Publication No. 2006-62002 (FIGS. 3 and 4 on pages 6 to 7).
In the general dicing method, the cutting speed and direction (up-cut or down-cut) are optimized to prevent chipping of the silicon material, and the blade is dressed at regular intervals, before significant blade wear develops, so that the diamond particles will not fall out and clog the kerf, where they can cause severe chipping.
These measures are, however, inadequate to prevent chipping in the dicing of wafers with specific surface orientations often required for MEMS products. For example, chipping has been found to occur in the dicing of bonded SOI wafers having a (100) silicon supporting layer several hundred micrometers thick and a (110) silicon active layer several micrometers thick, in which the crystal lattice orientations at the surfaces of the active layer and the supporting layer differ by 45 degrees. In this configuration, the thin silicon active layer is especially prone to chipping during dicing.
In a piezoresistive MEMS acceleration sensor of the type partly shown in FIG. 2, a resulting problem is that silicon flakes 71 that have chipped off may remain inside the sensor, in the gap 65 between the mass 22 and stopper 41a, for example, where they reduce the movable range of the mass or immobilize the mass, so that the sensor cannot operate properly or cannot operate at all.
A problem with the laser dicing method described above is that the MEMS design is constrained by the need to etch trenches in the supporting substrate layer below the scribe lines during the process of forming the MEMS structure.